Victorian health authorities have identified more than 10,000 people connected to positive coronavirus cases, sparking fears a cluster of 26 infections could explode.
The state will be plunged into a seven-day lockdown from 11.59pm tonight as health officials aim to stop the spread.
But Acting Premier James Merlino revealed a sobering number in his address to the media this afternoon - that more than 10,000 primary and secondary contacts would "need to either quarantine or test and isolate".
"That number will continue to grow and change," he said.
"Our public health experts' primary concern is how fast this variant is moving.
"We've seen overseas how difficult that movement can be to control. Here in Victoria, we're seeing not only how quick it is, but how contagious it is as well.
"Our contact tracers are identifying and locking down the first ring, second ring, and third ring within 24 hours. They have never done that before. That's the fastest our contact tracers have ever moved within a 24-hour period."
Chief health officer Professor Brett Sutton warned "any one of the [10,000] could become cases over the next 14 days".
He said the reason for the high number of primary contacts is because of the "generations of transmissions" which occurred before the first case in the outbreak was identified.
"There's been opportunities to spread to people. The other clear element is the infectiousness of the virus. When we see a member become infected, almost all the household is infected," he said.
"It speaks to the infectiousness of this B. 1.617 variant, but also the cycles it's going through. It really is rapid and that's led to the exponential increase."
Merlino said the time between catching the virus and passing it on was "tighter than ever".
"How long it takes between the onset of symptoms in the first and secondary case is averaging just over a day," he said.
"To put that in some perspective, the usual transmission is about five to six days – In some of these cases, within a day it's been transmitted."
The number of cases grew considerably in the past 24 hours after Victoria posted 11 new infections today.
There are now 34 active coronavirus cases across the state, with 26 of those linked to the new cluster. More than 40,000 test results were received overnight.
The number of sites visited by Covid-19 positive cases has soared past 100, with a McDonald's, Anytime Fitness gym and Kmart among the new venues.
Genomic testing has found the new infections are "closely linked" to a case from a few weeks ago that originated in South Australia.
Earlier in the month a man tested positive to the virus after becoming infected while undergoing hotel quarantine in South Australia and then flying into Melbourne and returning to his home in Wollert.
Health authorities are still struggling to find the "missing link" between this case and the new cluster, though so far none of the new cases have been found to be linked to any exposure sites from the Wollert case.
A man in his 60s, who was among the five cases announced on Tuesday, is still believed to be the earliest known infection in the new outbreak.
The man, referred to as case 5, developed symptoms on May 17, with authorities believing he may have been infectious for almost 10 days before presenting for testing.
Move to lockdown
In announcing the lockdown this afternoon, Merlino warned unless something "drastic happens", the situation would become "increasingly uncontrollable".
"The vaccine rollout has been slower than we have hoped," he said.
"It's not where we hoped it would be, it's not where it should be. If more people were vaccinated we might be facing a very different set of circumstances than we are today. But sadly we are not."
From 11.59pm tonight Victorians can only leave home for five reasons:
• Shopping for necessary goods and services
• Authorised work or permitted education
• Exercise – a 2-hour limit with one other person
• Care giving, compassionate, and medical reasons
• To get vaccinated
A whole raft of other safety measures were also announced in the "circuit breaker" lockdown, which will be in place until 11.59pm on June 3.
Essential retail - including supermarkets, food stores, petrol stations, banks, bottle shops and pharmacies - can open while other retail stores can provide click and collect service. Restaurants, pubs and cafes can provide takeaways only.
Schools will move to remote learning and public venues must close while public gatherings are banned. Masks must be worn outside the home.
Warning for New Zealand
A spokesperson from the New Zealand Ministry of Health said this afternoon it was "crucial" everyone who has been in Victoria since May 11 was checking Victoria Health's website for locations of interest.
Anyone who had been at a location of interest at the specified time must immediately self-isolate and call Healthline on 0800 358 5453.
Anyone with symptoms who had been in the wider Melbourne area since May 11 should also ring Healthline and arrange a test.
People who had been in the Whittlesea local government area since May 11 should also ring Healthline.
One case in ICU
Victorian authorities have confirmed a person who contracted coronavirus in the state has been placed into intensive care, as the state battles the highly infectious Indian variant.
Merlino said that the person in ICU was "not in a good way".
"In the last day, we have seen 12 linked new cases, bringing the total number of cases linked to the hotel quarantine breach in South Australia to 26," he said.
"Sadly, we have one of those people in an ICU, on a ventilator, in not a very good way."
Health Minister Martin Foley said that the patient in ICU was one of the first cases from the outbreak and an "elderly person".
He confirmed that they had not had the vaccine despite being over the age of 50.
Vaccine efforts expanded
Victoria has also expanded its vaccination programme in an attempt to combat the spread of the virus.
Merlino said from tomorrow, vaccinations at state sites would be expanded to include residents over the age of 40, who will be eligible to receive the Pfizer or AstraZeneca vaccines.
"If you're eligible, get vaccinated," he said.
"If we had an alternative to hotel quarantine for this particular variant of concern, we would not be here today.
"If we had the vaccine, the Commonwealth's vaccine programme effectively rolled out, we may well not be here today."
Aussie PM on the lockdown
Australian Prime Minister Scott Morrison spoke to reporters this afternoon saying Victorians again faced a "challenge and a battle".
"There are no certainties, no guarantees, in a global pandemic.
"The next seven days in Victoria will be very challenging ... once again we will come through."